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CSC271 Database Systems

CSC271 Database Systems. Lecture # 24. Summary: Previous Lecture. Classification of entities Attributes on relationships Structural constraints Multiplicity Binary/Complex relationships. Summary of Multiplicity Constraints. Structural Constraints.

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CSC271 Database Systems

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  1. CSC271 Database Systems Lecture # 24

  2. Summary: Previous Lecture • Classification of entities • Attributes on relationships • Structural constraints • Multiplicity • Binary/Complex relationships

  3. Summary of Multiplicity Constraints

  4. Structural Constraints • Multiplicity actually consists of two separate constraints known as • Cardinality • Participation

  5. Cardinality • Describes maximum number of possible relationship occurrences for an entity participating in a given relationship type • The cardinality of a binary relationship is what we previously referred to as a one-to-one (1:1), one-to-many (1:*), and many-to-many (*:*) • The cardinality of a relationship appears as the maximum values for the multiplicity ranges on either side of the relationship

  6. Participation • Determines whether all or only some entity occurrences participate in a relationship • The participation constraint represents whether all entity occurrences are involved in a particular relationship (referred to as mandatory participation) or only some (referred to as optional participation) • The participation of entities in a relationship appears as the minimum values for the multiplicity ranges on either side of the relationship • Optional participation is represented as a minimum value of 0 while mandatory participation is shown as a minimum value of 1

  7. Cardinality and Participation

  8. Problems with ER Models • Problems may arise when designing a conceptual data (creating an ER) model called connection traps • Normally occur due to a misinterpretation of the meaning of certain relationships • Two main types of connection traps are: • Fan traps • Chasm traps

  9. Connection Traps • In general, to identify connection traps we must ensure that the meaning of a relationship is fully understood and clearly defined • If we do not understand the relationships we may create a model that is not a true representation of the ‘real world’

  10. Fan Traps • Where a model represents a relationship between entity types, but pathway between certain entity occurrences is ambiguous • A fan trap may exist where two or more 1:* relationships fan out from the same entity as shown in the following example

  11. Fan Traps.. • At which branch office does staff number SG37 work?

  12. Resolving Fan Trap • Reconstruction of the original ER model to represent the correct association between these entities

  13. Fan Trap Resolved? • At which branch office does staff number SG37 work, same question again?

  14. Chasm Traps • Where a model suggests the existence of a relationship between entity types, but pathway does not exist between certain entity occurrences • A chasm trap may occur where there are one or more relationships with a minimum multiplicity of zero (that is optional participation) forming part of the pathway between related entities

  15. Chasm Traps.. • At which branch office is property PA14 available?

  16. Resolving Chasm Trap • Identify the missing relationship that represents the true association between entities

  17. Chasm Trap Resolved? • At which branch office is property PA14 available, same question again?

  18. Sample ER-Diagram

  19. Summary • Structural constraints • Multiplicity • Cardinality/Participation • Connection traps • Fan/Chasm traps

  20. References • All the material (slides, diagrams etc.) presented in this lecture is taken (with modifications) from the Pearson Education website : • http://www.booksites.net/connbegg

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